MEMBRANES.pptx itni yad you can do that for the

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1. MEMBRANES

2. Cell membrane/ Plasma membrane It is the boundary that separates the living cell from its non-living surroundings It is a thin film (6-10nm ) which can be only resolved with an electron microscope The PM is covered/surrounded by a cell wall in plant cells and by a cell coat in animal cells The plasma membrane exhibits selective permeability allowing some substances to cross it more easily than others The fluid mosaic model of membrane structures states that membrane is a fluid structure with a “mosaic” of various proteins embedded in a lipid bilayer. Cell membrane composition Proteins – 55% Lipids – 42% Carbohydrates – 3 % Human RBC: Proteins – 52%, Lipids – 40%, Carbohydrates – 8% Except myelin, all other cell membranes show a higher protein/lipid ratio

3. Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathic molecules , containing hydrophobic and hydrophilic regions • The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

4. Phospholipids • The phospholipids are arranged in a bilayer, with their polar , hydrophilic phosphate heads facing outwards • and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer

5. In 1972, Singer and Nicolson – Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer Phospholipid bilayer

6. • In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins • Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions • In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

7. Proteins Peripheral Proteins – located on either the intracellular or extracellular side of the cell, include hormone receptors Integral Proteins –are embedded in the lipid bilayer, span the entire membrane , include ion channels and transport proteins Functions of proteins: - Transport – Enzymatic activity – Signal transduction –– Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM)

8. Carbohydrates Functions : Protection Cell recognition Antigens such as the ABO antigens on blood cells

9. The Role of Membrane Carbohydrates in Cell-Cell Recognition • Cells recognize each other by binding to surface molecules , often containing carbohydrates, on the extracellular surface of the plasma membrane • Membrane carbohydrates may be covalently bonded to lipids ( forming glycolipids ) or more commonly to proteins (forming glycoproteins ) • Carbohydrates on the external side of the plasma membrane vary among species, individuals , and even cell types in an individual

10. HIV HIV can infect a cell that has CCR5 on its surface, as in most people. HIV cannot infect a cell lacking CCR5 on its surface, as inresistant individuals. Receptor (CD4) Co-receptor (CCR5) Receptor (CD4) but no CCR5 Plasma membrane

11. Membrane Models: Scientific Inquiry Membranes have been chemically analyzed and found to be made of proteins and lipids Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer

12. • In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins • Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions • In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

13. Fluid Mosaic Model of membranes – membrane proteins are dispersed and individually inserted into the phospholipid bilayer Phospholipid bilayer

14. Freeze-fracture studies of the plasma membrane supported the fluid mosaic model • Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer

15. Function of cell membranes Forms boundary between the contents of the cell and its external environment. Regulates the passage of substances into and out of the cell (certain substances). Enables the cell to communicate with the external environment (receptor proteins that receive chemical messenger molecules e.g hormones from other cells).

16. The Permeability of the Lipid Bilayer • Hydrophobic (nonpolar) molecules, such as hydrocarbons , can dissolve in the lipid bilayer and pass through the membrane rapidly • Polar molecules, such as sugars, do not cross the membrane easily Transport Proteins • Transport proteins allow passage of hydrophilic substances across the membrane • Some transport proteins, called channel proteins , have a hydrophilic channel that certain molecules or ions can use as a tunnel • Channel proteins called aquaporins facilitate the passage of water Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane • A transport protein is specific for the substance it moves

17. Effects of Osmosis on Water Balance • Osmosis is the diffusion of water across a selectively permeable membrane • Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides

18. Water Balance of Cells Without Walls • Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water • Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane • Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water • Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

19. Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O H2O H2O H2O H2O Lysed Normal Shriveled Turgid (normal) Flaccid Plasmolyzed Animal cell Plant cell

20. • Hypertonic or hypotonic environments create osmotic problems for organisms • Osmoregulation , the control of solute concentrations and water balance, is a necessary adaptation for life in such environments • The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Contractile vacuole

21. Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm ) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt In a hypertonic environment, plant cells lose water ; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

22. Membrane Transport Movement of molecules / substances in and out of cell Cell membranes are a barrier to most substances , and this property allows materials to be concentrated inside cells, excluded from cells, or simply separated from the outside environment.

23. Methods by which substances can move across a cell membrane: Passive Transport (simple diffusion, facilitated diffusion and osmosis ) Active Transport. Bulk transport (Exocytosis and Endocytosis )

24. Movement of substances in and out of cell

25. Passive transport Passive transport is the transport of substances by a trans-membrane protein molecule or through lipid bilayer. No energy required Move due to gradient

26. Simple Diffusion A few substances can diffuse directly through the lipid bilayer part of the membrane (such as H2O, O2 and CO2 ) Small water-soluble molecules can pass through channels .

27. Factors affecting Diffusion The rate of diffusion directly proportional to: 1. Concentration gradient. 2. Solubility in the membrane. 3. Temperature. 4. Surface area of the membrane . The rate of diffusion inversely proportional to: 1. Molecular weight of diffusible substance. 2. Thickness of the membrane.

28. Facilitated Diffusion Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion , transport proteins speed the passive movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include – Aquaporins , for facilitated diffusion of water – Ion channels that open or close in response to a stimulus ( gated channels )

29. Facilitated diffusion It is similar to simple diffusion in the sense that it does not require energy and transport is down an electrochemical gradient.

30. Types of Transport Proteins Channel proteins Provide corridors that allow a specific molecule or ion to cross the membrane A channel protein (purple) has a channel through which water molecules or a specific solute can pass Channel protein Solute EXTRACELLULAR FLUID CYTOPLASM

31. Carrier proteins – Undergo a subtle change in shape that translocate the solute-binding site across the membrane A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

32. Active transport • Facilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energy • Some transport proteins, however, can move solutes against their concentration gradients. Requires energy, usually supplied by ATP Active transport is performed by specific proteins embedded in the membranes Active transport allows cells to maintain concentration gradients that differ from their surroundings • The sodium-potassium pump is one type of active transport system

33. Passive transport Active transport Diffusion Facilitated diffusion ATP

34. How Ion Pumps Maintain Membrane Potential • Membrane potential is the voltage difference across a membrane • Voltage is created by differences in the distribution of positive and negative ions across a membrane Two combined forces, collectively called the electrochemical gradient , drive the diffusion of ions across a membrane – A chemical force (the ion’s concentration gradient ) – An electrical force (the effect of the membrane potential on the ion’s movement ) An electrogenic pump is a transport protein that generates voltage across a membrane • The sodium-potassium pump is the major electrogenic pump of animal cells • The main electrogenic pump of plants, fungi, and bacteria is a proton pump • Electrogenic pumps help store energy that can be used for cellular work

35. CYTOPLASM EXTRACELLULAR FLUID ATP Proton pump H + H + H + H + H + - - - -

36. Cotransport : Coupled Transport by a Membrane Protein Co-transport occurs when active transport of a solute indirectly drives transport of other solutes • Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

37. ATP Proton pump Sucrose-H + cotransporter Sucrose Sucrose Diffusion of H + H + H + H + H + H + H + H + - - + +

38. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins Large molecules, such as polysaccharides and proteins , cross the membrane in bulk via vesicles Bulk transport requires energy

39. Exocytosis • In exocytosis , transport vesicles migrate to the membrane , fuse with it, and release their contents • Many secretory cells use exocytosis to export their products Endocytosis • In endocytosis , the cell takes in macromolecules by forming vesicles from the plasma membrane • Endocytosis is a reversal of exocytosis, involving different proteins • There are three types of endocytosis – Phagocytosis (“cellular eating”) – Pinocytosis (“cellular drinking”) – Receptor-mediated endocytosis

40. In phagocytosis a cell engulfs a particle in a vacuole. The vacuole fuses with a lysosome to digest the particle. In pinocytosis , molecules are taken up when extracellular fluid is “gulped” into tiny vesicles. In receptor-mediated endocytosis , binding of ligands to receptors triggers vesicle formation. • A ligand is any molecule that binds specifically to a receptor site of another molecule

41. Phagocytosis Pinocytosis Receptor-Mediated Endocytosis Solutes Pseudopodium “Food” or other particle Food vacuole CYTOPLASM Plasma membrane Vesicle Receptor ligand Coated pit Coated vesicle Coat proteins

42. An amoeba engulfing a bacterium via phagocytosis (TEM). Phagocytosis EXTRACELLULAR FLUID Pseudopodium Solutes Food” or other particle Food vacuole Bacterium Food vacuole Pseudopodium of amoeba

43. Pinocytosis vesicles forming in a cell lining a small blood vessel (TEM). Pinocytosis Vesicle

44. Receptor-Mediated Endocytosis Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis (TEMs). Plasma membrane Coat proteins

45. INTERCELLULAR JUNCTIONS Inter-cellular junctions are f undamental to the interactions between cells A llow activities of individual cells that make up the systems to be co-ordinated Enable each system to function as an integrated system Animals versus Plants I ntercellular junctions in animals: Tight Junctions Gap Junctions Desmosomes Adherens Junctions Plants : P lasmodesmata

46. Tight Junctions These are regions in which two cells are very tightly connected together, and they will prevent some molecules from passing across an epithelium. The borders of two cells are fused together, often around the whole perimeter of each cell, forming a continuous belt like junction known as a tight junction or zonula occludens ( zonula = latin for belt). The permeability of tight junctions varies from site to site, they can often be selectively leaky. For example, these junctions are important in the gut, in acting as a selective diffusion barrier, preventing diffusion of water soluble molecules. They also act to restrict the localisation of membrane bound proteins .

47. Gap Junctions Also called “Communicating Junctions” 2 opposing connexons join across intercellular space Connexons : assembly of six proteins that create gap between two plasma membranes

48. A group of protein molecules called connexins form a structure called a connexon When connexons from two adjacent cells align , they form a continuous channel between them . This channel is big enough to allow small molecules such as inorganic ions, and other small water soluble molecules (smaller than 1000kDa) to pass between the cells. However the channel is too small for proteins, nucleic acids or sugars .

49. Functions of Gap Junctions Gap junctions are the most widespread of all cell junctions in animal tissues. F orm in a narrow gap of 2-4nm, between two adjacent cells. C ouple cells electrically and metabolically, enabling cells to communicate with each other directly. C an open and close in response to changes in calcium levels, and pH.

50. The picture shows an EM (A) of two gap junctions between two cells, and a freeze fracture EM (B) of the particles on the cytoplasmic face of the plasma membrane.

51. Desmosomes Desmosomes  connect two cells together. A lso known as a spot desmosome or  macula adherens  (macula = latin for spot), because it is circular or spot like in outline, and not belt- or band shaped like adherens junctions. Also called “Anchoring Junctions” Arranged randomly on the lateral side of cell membranes The adhesion protein bridges the space between the cells Particularly common in epithelia that need to withstand abrasion.

52. This diagram shows a  desmosome . It is made up of a dense cytoplasmic plaque, to which the intermediate filaments attach.

53. The Function of Desmosomes Fasten cells together into strong sheets Attach muscle cells to each other in a muscle ( Muscle tears can involve rupture of desmosomes)

54. Summary of Junctions in Animals

55. Plasmodesmata (In Plants) Every living cell in a higher plant is connected to its living neighbors by fine cytoplasmic channels, each of which is called a plasmodesma which passes through the intervening cell walls . They act like tunnels running through the cell wall which allow communication with the other cells in a tissue . The plasma membrane of one cell is continuous with that of its neighbor at each plasmodesma . They are formed around the elements of the smooth endoplasmic reticulum that become trapped during cytokinesis (of mitotic cell division) within the new cell wall that will bisect the parental cell. Here the wall is not thickened further, and depressions or thin areas known as pits are formed in the walls. Pits normally pair up between adjacent cells. They can also be inserted into existing cell walls between non-dividing cells (secondary plasmodesmata ). They occur in varying numbers.

56. Structure of Plasmodesmata They are roughly cylindrical, membrane-lined channels with a diameter of 20 to 40 nm. Constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the  desmotubule . Running from cell to cell through the center of most plasmodesmata is a narrower cylindrical structure, the desmotubule , which remains, continuous with elements of the SER membranes of each of the connected cells. Between the outside of the desmotubule and the inner face of the cylindrical  plasma membrane  is an annulus of cytosol, which often appears to be constricted at each end of the plasmodesmata . These constrictions may regulate the flux of molecules through the annulus that joins the two cytosols. The plasma membrane portion of the plasmodesma is a continuous extension of the cell membrane or  plasmalemma  and has a similar phospholipid bilayer structure.

57. Functions of Plasmodesmata They are narrow channels that act as intercellular cytoplasmic bridges to facilitate communication and transport of materials between plant cells. They serve to connect the symplastic space in the plant and are extremely specialized channels that allow for intercellular movement of water, various nutrients, and other molecules. Plasmodesmata function in intercellular communication, i.e., they allow molecules to pass directly from cell to cell. P lasmodesmata mediate transport between adjacent plant cells, much as gap junctions of animal cells. They allow the passage of molecules with molecular weights of less than 800 daltons . Plasmodesmata have been shown to transport proteins (including transcription factors), short interfering RNA, messenger RNA,  viroids , and viral genomes from cell to cell. Plasmodesmata are also used by cells in the phloem, and symplastic transport is used to regulate the sieve-tube cells by the companion cells .